Transformer Winding resistance test
We will show in this article one of transformer testing. Transformer winding resistance test…
We explain before types of transformer tests .. there were 6 types of transformer testing .. you can check these types at this link … https://www.top-ee.com/transformer-testing
Purpose of transformer winding resistance test:
This test is performed on the transformer to:
- Check continuity of all internal connections.
- Check loose connections of bushing or tap changer that may cause high contact resistance.
Precautions before do this test:
- Disconnect the transformer electrically.
- Clean the terminals of transformer.
Steps for transformer winding resistance test :
Firstly we Measure the resistance value of high voltage windings:
- Measure resistance between each phase with another phase as shown in previous fig.
- Repeat this test at all tap changer points.
- These values of resistances should be same and identical with the nameplate value recorded on the transformer with error permeability of 5% maximum, if higher than 5%, transformer test fails.
Secondly we measure the resistance value of Low voltage windings:
- Measure resistance between each phase and another phase as shown in previous fig.
- No tap changer points at Low voltage side.
- Measured values should be identical with the nameplate value of the transformer
But when measuring the resistance of low voltage winding in this way we do not get accurate results because of the small value of resistance to low voltage winding so, In this case, we can use any resistance bridge law. For example, the famous Wheatstone Bridge, where a known R1 and R3 resistors are used to measure the resistance of an unknown Rx by changing the value of the R2 resistance until we reach the equilibrium point between the two branches. Then the current in the Galvanometer is zero and here we apply the following rule:
Rx = R2 * (R1/R3)
For more accurate testing, we must take the temperature of the surrounding medium into consideration as the resistance value depends on the temperature. Therefore, we must correct the measured value during the test according to the following equations